1. Field of the Invention
This invention generally relates to an oscillating circuit and more particularly is directed to an oscillating circuit suitable for use in a frequency converter for converting or changing the center frequency of a frequency modulated signal.
2. Description of the Prior Art
An oscillator for providing a single signal used in, for example, a frequency converter circuit, is generally in the form of a Colpitts oscillator which employs a quartz oscillator as shown in FIG. 1.
FIG. 1, reference letter Qa represents an amplifying transistor, C.sub.1 and C.sub.2 are capacitors for determining a feedback capacity and X is a quartz vibrator or crystal resonator.
When the single signal to be used in the above mentioned signal converter circuit contains a higher harmonic component of other than the fundamental frequency, that component becomes spurious and affects the signal converter system. Therefore, it is desired that the single signal be limited to a single mode as much as possible.
However, since a Colpitts oscillator such as is shown in FIG. 1 at 20 contains a signal component of high-order mode, it is difficult to produce a desired single signal component. Also, when the oscillating or resonance frequency f.sub.c is relatively low, the capacities of the capacitors C.sub.1 and C.sub.2 become relatively large. As a result, when the Colpitts oscillator 20 is formed as an IC (integrated circuit), it is necessary for the capacitors C.sub.1 and C.sub.2 to be externally connected thereto. Therefore, in that case, at least two external terminals 19a and 19b are required.
Further, in order to reduce as much as possible the distortion of the single signal Sc which is the oscillating output, it is necessary to accurately adjust the feedback capacities of the capacitors C.sub.1 and C.sub.2. This adjustment is quite troublesome. Furthermore if in order to obtain a constant feedback capacity, a resistor R.sub.E connected to the emitter of the transistor Qa is controlled in response to the output of the single signal Sc obtained at an output terminal 20a, the resistor R.sub.E can not be controlled without difficulty if the circuit is formed as an IC.
Therefore, with the conventional circuitry shown in FIG. 1, it is impossible to easily obtain an oscillating output having low distortion.